The surprising discovery that blood and other body fluids harbor abundant and stable extracellular microRNAs (ex-miRNAs) set off a boom of studies to exploit them for diagnostic purposes and to understand their function in health and disease. To realize the potential of this research opportunity, studies are urgently needed to fill gaps in our knowledge regarding the source cells that secrete ex-miRNAs, the forms in which they are stabilized in body fluids, the regulation and specificity of their secretion, and the functional effects of ex-miRNAs in both source and target cells. In this project, we will test the central hypothesis that immune cells release ex-mi RNAs in response to inflammatory stimuli, and that this process is critical for their immune function. Published findings suggest that circulating immune cells may be the biggest source of ex-miRNAs in blood. In Specific Aim 1, we will use mouse genetic models of immunodeficiency and protocols for immune cell depletion together with a high throughput qRT-PCR platform for ex-mlRNA profiling to define the immune cell contribution and extracellular form of blood and airway lining fluid ex-miRNAs. These studies will be extended to mouse models of asthma in Aim 2 to determine how inflammation regulates ex-mlRNA release from immune cells. The second aim will also encompass in vitro experiments that will define the form and molecular pathway of T cell ex-miRNA and Ago2 secretion in response to antigen-driven activation. And in Aim 3 we will examine the functional consequences of this induced ex-miRNA release for the source T cells. Specifically, we will determine whether ex-miRNA secretion is involved in the global down regulation of cellular miRNAs and Ago proteins and rapid resetting ofthe miRNA repertoire that supports activated T cell proliferation and differentiation into immune effector cells.